1. Field of the Invention
The present invention generally relates to optical semiconductor devices, and more particularly to an optical semiconductor device having a periodic layer structure. Further, the present invention is concerned with a method of producing such an optical semiconductor device, and a laser device using optical semiconductor devices as described above.
2. Description of the Prior Art
As is well known, semiconductor materials have respective inherent bandgaps and refractive indices. Generally, a semiconductor material has the function of absorbing light having a wavelength equal to or less than the bandgap thereof. An optical confinement effect takes place by surrounding a semiconductor material having a high refractive index by another semiconductor material having a low refractive index. By utilizing the optical confinement effect, an optical waveguide structure can be configured.
A periodic structure in which the refractive index varies periodically in a propagation direction of light functions as a grating. For example, grooves are photolithographically formed at fixed intervals (a fixed period) on a surface of a substrate or a buffer layer formed on the substrate. A semiconductor layer having a refractive index different from that of the substrate or the buffer layer is formed on the surface thereof so that the grooves are buried by the semiconductor layer. A waveguide is formed on the semiconductor layer. An optical semiconductor device thus formed functions as a grating.
A direct transition type semiconductor has a luminescent recombination effect by injecting current into the semiconductor. By using the above effect, an intrinsic layer of a pin diode is made of a direct transition type semiconductor, and a resonator structure having the above pin diode structure serves as a semiconductor laser.
A distributed feedback (DFB) laser structure having a fixed oscillation wavelength can be made by a combination of a semiconductor laser and a grating. The grating functions as a selective mirror with respect to a light having the fixed oscillation wavelength, and oscillates with a wavelength in which lights reflected by grooves of the grating are in phase with each other.
As described above, semiconductor materials function not only as passive optical media having respective fixed refractive indexes but also as light absorbing media or light emitting media.